1. Field of the Invention
This invention relates to a level gauge for detecting a level of liquid helium which is accomodated in a container made of metals, glasses or other materials. More particularly, the invention relates to a level gauge for detecting a level of liquid helium which makes use of, as a sensing element, a wire made of an amorphous superconductive alloy obtained by rapid quenching of a molten alloy material and in which the level of liquid helium is detected by measurement of an electric resistance of the sensing element.
2. Description of the Prior Art
Masumoto and Inoue disclosed, in Japanese Patent Application No. 57-049911 filed Mar. 26, 1982 and now laid-open in Japanese Laid-open Patent Application Gazette No. 58-166220, level gauges which make use of wires, made of amorphous superconductive alloys, as sensing elements in order to detect a level of liquid helium.
The amorphous superconductive alloys used to make the sensing elements of the level gauge are alloys which are obtained by rapid quenching of molten alloy materials and which comprise 20% by volume or more of an amorphous phase and whose superconducting transition temperature, Tc, ranged from 4.2 to 5.0.degree. K. The alloys are represented by the general formula, Z.sub.a.M.sub.b.(Q+Al).sub.c, in which Z is a member selected from the group consisting of Zr, Hf and Ti, M is a member selected from the group consisting of V, Nb and Ta, Q is Si or Ge, a is a value of 10 to 90 atomic %, b is a value of less than 80 atomic %, and c is a value of 10 to 25 atomic %.
The level gauge disclosed in the above patent application utilizes a wire sensing element of the superconductive alloys. The level gauge is attached to a container in such a way that the sensing element is vertically disposed into liquid helium accomodated in the container. The level of the liquid helium is detected as follows: the sensing element is connected to a D.C. power supply at opposite ends thereof and applied with a small electric current to measure a variation of an electric resistance of the element.
Of the amorphous superconductive alloys represented by the above general formula, Z.sub.a.M.sub.b.(Q+Al).sub.c, have the following problems to be solved when applied as wire sensing element of a level gauge. Because of the addition of Nb to Zr which is a main ingredient of the superconductive alloy of the above formulation, the melting temperature of the alloy becomes very high, i.e. about 1800.degree. C., so that specific types of equipments or apparatus are needed to melt the alloy.
In the U.S. Patent Application Ser. No. 06/673,931 now U.S. Pat. No. 4,655,079 of Masumoto et al, filed Nov. 21, 1984, there is described a level gauge for liquid helium which includes a sensing element made of a superconductive alloy wire which has a superconducting transition temperature, Tc, of 4.2 to 4.5.degree. K., a support member for linearly supporting and dielectrically insulating the sensing element, means for energizing the sensing element and means for detecting a current passed through the sensing element. The superconductive alloy consists essentially of Zr, Ru and Rh which are represented by the following formula, EQU Zr.sub.100-x (Ru.sub.y.Rh.sub.1-y).sub.x
in which x represents a content of at least one of Ru and Rh in atomic % and is in the range of 22.5.ltoreq..times..ltoreq.27.5 and y is a value of 0.ltoreq.y.ltoreq.1. The superconductive alloy has an amorphous phase which is obtained by rapid quenching of a molten alloy material and whose maximum transition temperature, Tc, is about 4.5.degree. K.
In FIG. 1, there is shown the relationship between the temperature of liquid helium and the vapor pressure. As will be seen from the graph, the temperature of liquid helium increases with an increase of the pressure (vapor pressure). At about 1.3 bars or higher, the temperature of liquid helium is higher than 4.5.degree. K. This means that the sensing element made from the above superconductive material does not exhibit the superconducting transition in liquid helium at a pressure over 1.3 bars. Therefore, a level gauge using the sensing element cannot be applied to a container accomodating liquid helium in an atmosphere where the pressure is over 1.3 bars. In other words, the level gauge cannot be used as a gauge for monitoring the level of liquid helium at the time when liquid helium is transferred using the container or is charged into the container. As will be clearly seen from FIG. 1, the critical temperature of liquid helium is 5.2.degree. K., i.e. the temperature at which helium can be liquid is below 5.2.degree. K. Accordingly, there is a demand of a level gauge for liquid helium which has a sensing element made from a superconductive material having superconducting transition temperature of from 4.5.degree. of 5.2.degree. K.